The present invention is a process for separating a gaseous mixture comprising C.sub.1 chlorocarbons and noncondensible gases into a liquid component comprising the C.sub.1 chlorocarbons and a component comprising the non-condensible gases. The method employs a liquid hydrocarbon having an average molecular weight within a range of about 142 to 422 to adsorb the C.sub.1 chlorocarbons from the gaseous mixture. The present process is especially useful for separating methyl chloride from a gaseous mixture resulting from an oxychlorination process where the gaseous mixture further comprises, methane, water vapor, and hydrogen chloride.
It is known to chlorinate methane by a process typically termed "oxychlorination." In processes of this type, gaseous hydrogen chloride and an oxygen containing gas such as air and the hydrocarbon to be chlorinated are contacted with a metal halide catalyst which may also comprise stabilizers and promoters. By a series of well known reactions, elemental chlorine (Cl.sub.2) is released from the hydrogen chloride and chlorinates the hydrocarbon feed material. In another modification of this process, elemental chlorine (Cl.sub.2) is used as the feed gas in place of gaseous HCl. This latter process operates in a manner similar to the first except that an initial chlorination of hydrocarbon takes place. Thus, free chlorine, and oxygen containing gas, and the hydrocarbon to be chlorinated are contacted with the metal halide catalyst. The chlorine reacts with the hydrocarbon to produce hydrogen chloride and a chlorinated product of the hydrocarbon. Hydrogen chloride produced in this manner is then converted to elemental chlorine by a well known series of reactions, thereby providing additional chlorine for the chlorination of more hydrocarbon feed.
Although oxychlorination processes of this type are well known in the art, there are serious operational difficulties generally associated with them. For example, it is found that serious difficulty arises in the recovery of the chlorinated hydrocarbon products produced by such reactions. This is due in part to the fact that the chlorinated hydrocarbon products are diluted in great quantities of inert or noncondensible gases such as methane, elemental nitrogen, carbon monoxide, carbon dioxide, and other like gases. In order to recover the products satisfactorily from such a process it is necessary to process large quantities of gas and efficiently recover the chlorinated hydrocarbon content thereof.
The recovery process is further complicated by the presence of water and hydrogen chloride in such mixtures which can condense to form an aqueous hydrogen chloride solution. This aqueous hydrogen chloride solution can not only be detrimental to process equipment but may also have deleterious effects on solvents used in separation processes.
Deim et al., U.S. Pat. No. 3,148,041, describe a process where a gaseous mixture containing chlorinated methanes and predominating quantities of noncondensible gaseous components is contacted with a liquid aromatic halogenated hydrocarbon after scrubbing with an aqueous alkaline solution to remove hydrogen chloride. Deim et al. report that with this process it is possible to absorb essentially all of the chlorinated methane content of such a mixture while permitting the non-chlorinated methane and noncondensible gases to pass through the absorbent.
Taso, U.S. Pat. No. 4,039,597, teaches that chlorinated hydrocarbons may be separated from a gas stream containing unreacted hydrocarbon, carbon dioxide, and chlorinated hydrocarbons by such processes as condensation and fractionation.
Sisson, U.S. Pat. No. 4,020,117, describes a process for recovering methyl chloride and methylene chloride from the effluent of an oxychlorination process. The process comprises cooling the effluent and contacting with an absorbent that is specific for the methyl chloride and dichloromethane. The absorbent is then stripped with methane to recover the methyl chloride and dichloromethane.
Taso et al., U.S. Pat. No. 4,193,944, describe a process where a purge stream containing inerts, unreacted alkane and chlorinated hydrocarbon is recovered from a chlorinated hydrocarbon production effluent. The purge gas is contacted with an absorption oil recovered from the effluent which is at least one chlorinated hydrocarbon boiling at a temperature of at least 140.degree. C. to absorb chlorinated hydrocarbon.
The present process offers advantages over the described prior art processes for recovering C.sub.1 chlorocarbons from a gaseous mixture with noncondensible gases. The present process employs a liquid hydrocarbon having an average molecular weight within a range of about 142 to 422 to adsorb the C.sub.1 chlorocarbons from the gaseous mixture. The described liquid hydrocarbon is stable at elevated temperatures in the presence of aqueous hydrogen chloride. Therefore it is not necessary to treat the gaseous mixture to neutralize, as taught by Diem et al., supra, when the adsorption media is a halogenated aromatic hydrocarbon. The described process allows large volumes of gases containing less than about 25 percent C.sub.1 chlorocarbons to be processed more economically to recover the C.sub.1 chlorocarbons than can be achieved by methods such as condensation and fractionation as suggested by Taso, supra. In addition, the adsorbed C.sub.1 chlorocarbons can be stripped from the described liquid hydrocarbon with very little carryover, allowing the liquid hydrocarbon to be continuously recycled to the process. Furthermore, use of the liquid hydrocarbon avoids environmental issues associated with the use of halogenated hydrocarbons.